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| IMPROVE PRODUCTIVITY DURING WELDING OF STRAIGHT CHROME STAINLESS STEELS - PART II: FERRITIC STAINLESS STEELS |
| In the last issue we had detailed the welding of martensitic stainless steels. In this issue we will discuss the welding of ferritic stainless steels which are also a part of the straight chrome stainless steels. |
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| WHAT ARE FERRITIC STAINLESS STEELS? |
| Ferritic stainless steels are basically iron carbon alloys containing chromium in the range of 17-30% and relatively low carbon levels. The presence of high percentage of chromium and low percentage of carbon prevents the formation of austenite at elevated temperatures. Because of this, the room temperature structure remains ferritic. |
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| WHAT ARE THEIR UNIQUE PROPERTIES? |
The ferritic stainless steels have the following properties in general.
- Ferritic structure at room temperature
- Magnetic and hence attracted by magnets
- Non-hardenable and so during cooling lesser tendency to crack
- Poor toughness, in general
- Excellent resistance to chloride stress corrosion cracking, resistance to pitting and crevice corrosion in chloride media, resistance to oxidation at high temperature
- Excellent elevated temperature properties
- Poor weldability
The above will show that ferritic grades have unique properties because of which they become the candidate material for certain critical applications like stress corrosion service etc. Advanced ferritic grades are also available with improved weldability and other properties. |
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WHAT ARE THE POPULAR GRADES AND THEIR
COMPOSITIONS? |
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| GRADE | TYPICAL COMPOSITION |
| 430 | 0.06C-17Cr |
| 446 | 0.10C-25Cr |
| 405 | 0.05C-14Cr-0.2Al |
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| THEIR WELDING BEHAVIOR |
As indicated above, the ferritic grades contain large amounts of chromium which renders the formation of austenite impossible even at high temperatures. So during heating and cooling the stainless steel remains ferritic throughout. No transformation to hard phases and so no pre-heating etc. But there can be other issues which require careful evaluation.
- Grain coarsening and subsequent loss of toughness:This happens due to welding heat and also because of the poor conductivity of stainless steels; control of heat input is therefore essential. In base materials grain refiners like Ti, Al is also added to restrict the coarsening effect
- 475°C embrittlement: This happens when these steels are exposed to temperature ranges of 400-500C and this reduces the toughness of the material. Controlling the heat input can help in reducing this embrittlement
- Formation of sigma phase: Long soaking periods in the temperature range of 540-850C can produce sigma phase which is a hard and brittle phase. It not only affects the toughness but also the corrosion properties
- Notch sensitivity: With higher percentages of Cr, the material becomes brittle
- High temperature embrittlement: This happens when the steel is cooled rapidly from 1100C which is because of the super saturation of ferrite with carbon
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| WELDING PROCESSES AND SELECTION OF CONSUMABLES |
| For welding of ferritic stainless steels processes like SMAW, SAW, GTAW and GMAW are suitable. SMAW is very popular and the other processes like GMAW and GTAW are used for superior grades of ferritic stainless steels. Ferritic stainless steels can be welded with matching composition or with austenitic type welding consumables. |
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| GRADE | WELDING CONSUMABLES |
| 430, 405, | E430 or E309 |
| Ferritic grades to MS | E309 |
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| Both types of welding consumables have some advantages and some disadvantages as detailed below. |
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| MATCHING COMPOSITION WELD METAL | AUSTENITIC TYPE WELD METAL |
| Matching corrosion properties | Corrosion properties will not match |
| Toughness will require improvement through alloy additions, grain refining, post weld annealing heat treatment etc. | Toughness is good |
| Less distortion | More distortion |
| Embrittlement possibility | No possibility of embrittlement |
| Not suitable for Cr percentages above 23% | Suitable for higher percentages of Cr |
| Less costly than austenitic type | Costly compared to ferritic type |
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| PREHEAT AND INTER PASS TEMPERATURES |
| Normally ferritic grades can be welded without preheat. However, when the Cr content increases, preheat becomes essential. For steels containing more than 25% Cr, preheat and inter pass temperature of 150-200°C is employed. In case of thicker plates, again preheat is required. |
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| WELDING PROCEDURE |
The welding procedure should take care of the following aspects:
- Use basic coated electrodes when selecting matching consumables
- Use of minimum heat input
- Lowest size of electrode possible should be used
- Weld with Stringer beads and shortest possible arc
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| POST WELD HEAT TREATMENT |
| Ferritic stainless steels are normally subjected to a variety of heat treatments, when a matching filler metal is used, for a variety of purposes like grain refinement, improving toughness, removing embrittlement etc. There are guidelines available for choosing them and proper selection should be made after consulting these guidelines. |
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| ADOR WELDING LTD. ELECTRODES |
| We have in our range both matching and austenitic type of electrodes for welding ferritic grades of stainless steels. They are as given below. Please click on the brand name to know more about the electrodes. |
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| CONCLUSION |
Welding of ferritic stainless steels is an important activity in many industries since these materials are used in some critical service conditions. A good understanding of the behavior of this material will help in choosing the correct consumable and welding procedure which is essential for achieving tough, corrosion resistant weldments.
Please contact us at cmo@adorians.com for suggestions on selecting consumables and pre and post weld treatment for ensuring defect free welds to increase productivity. |
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